Automated friction clutches in the drivetrain of motor vehicles and methods to control them are known. High loads, especially on an incline and/or when a trailer is used, can cause the maximum transmission capacity to he exceeded. The friction clutch consequently overheats and the friction linings are destroyed, so that the motor vehicle is without any safety mechanism. Furthermore, component tolerances can cause insufficient pressure to be provided, or the coefficient of friction of the friction linings and/or their opposing friction surfaces to be too low, when friction clutches are compressed and then generate increasing clutch torque by an axial load on the clutch lever. Furthermore, in addition to destroying the friction lining, heating it lowers the coefficient of friction, and a less aggressive treatment of the heated friction linings can protect them from being destroyed.
Various measures have therefore been pursued to prevent the friction clutch from being overheated, especially the friction lining of the clutch disc. When friction clutches are applied, the friction clutch is engaged or disengaged when the specified temperature limits are exceeded to notify the driver of the situation of the friction clutch and cause the friction clutch to be cooled.
Another option is to limit the engine torque which, in an extreme case, can cause the internal combustion engine to be deactivated. As disclosed in German Patent No. 19752276 A1,, the engine torque is limited as the speed of the internal combustion engine is monitored in the specified driving situations, for example, while starting up. If the speed is exceeded, the engine torque is restricted to a specified value, and the restriction is lifted when the speed falls below the limit. This can cause uncomfortable alternation between torque limitation and its release.
A protective strategy for an automatic friction clutch is known from German Patent Application No. 10 2009 014 467, A1. When the friction clutch experiences slip, the engine torque is reduced until the driver requires greater torque. A new control loop is run. through, and the driver is warned, if applicable. It is therefore not possible to specifically lower the engine torque to protect the friction clutch independent of the driver's required torque. If, for example, the driver continuously requires greater torque than the reduced engine torque, he will be warned if it cannot be provided. However, when the driver's required torque remains below the reduced engine torque, it is not independently increased, and consequently a triggered increase or restoration of the driver's required torque is not immediately available when greater engine torque is suddenly demanded. Furthermore, the routine is suspended for limiting the engine torque when the driver's desired torque is lower than the limited engine torque, and a new slip control is activated that can cause the friction clutch to heat up and the transmissible clutch torque to be reduced.
The commonality of these procedures is that the friction clutch, which can only transmit limited torque in comparison to its maximum transmissible torque in its current operating state, for example, due to an elevated operating temperature, dirt, etc., is additionally negatively influenced by the review of its operability and is sometimes further damaged. To effectively protect the friction clutch and prevent additional checking routines which continue to generate friction clutch slip and further enhance the fault, the identified fault (fading) is therefore necessarily sustained with less transmissible torque, and therefore limited engine torque, until perhaps the internal combustion engine is turned off. This is associated with a loss of comfort to the driver since he has to dispense with some of the performance of the internal combustion engine, even though the friction clutch is capable of transmitting its maximum torque.